Preparation

Sauté pancetta in heavy large saucepan over medium-high heat until crisp, about 5 minutes. Using slotted spoon, transfer pancetta to paper towels.

Add 1 1/2 tablespoons butter to drippings in same saucepan; add shallots and reduce heat to medium. sauté until shallots are soft and golden, about 4 minutes. Stir in 1 teaspoon rosemary, then mashed sweet potatoes and 31/2 cups broth. Bring to boil. Reduce heat to medium-low and simmer 10 minutes to blend flavors, adding more broth by 1/4 cupfuls to thin soup, if desired. Season to taste with salt and pepper. Puree soup in blender or processor if necessary.